The present invention relates generally to magnetic resonance imaging (MRI) and more particularly, to a system and method to selectively create separate venous and arterial blood flow images for use in MRI angiography.
Quantitative flow data can readily aid in the diagnosis and management of patients and also help in the basic understanding of disease processes. There are many techniques available for the measurement of blood flow, including imaging based methods using radiographic imaging of contrast agents, both in projection and computed tomography (CT), ultrasound, and nuclear medicine techniques. Radiographic and nuclear medicine techniques require the use of ionizing radiation and/or contrast agents. Some methods involve making assumptions about the flow characteristics which may not necessarily be true in vivo or require knowledge about the cross-sectional area of the vessel or the flow direction. Ultrasound techniques are difficult to apply in certain situations because of intervening bone or air.
MRI utilizes radio frequency pulses and magnetic field gradients applied to a subject in a strong magnetic field to produce viewable images. Contrast agents are used to improve MR images. Such contrast agents include magnetizable substances having metals or metallic compounds. Such contrast agents may be paramagnetic, ferromagnetic, or superparamagnetic and act through dipole interactions with tissue protons. Most MR imaging contrast agents have similar mechanisms of action. Most are based on gadolinium chelates and therefore, are paramagnetic agents that develop a magnetic moment when placed in a magnetic field.
With the increasing use of MR contrast agents for MR angiography (MRA), arterial and venous signals become equally enhanced. The reduced T.sub.1 relaxation time removes the possibility of using spatial saturation RF pulses to eliminate either arterial or venous signals for flow discrimination. Automatic bolus detection merely addresses this issue by triggering the acquisition only when the flow is in the arterial phase. However, with the contrast already on board, subsequent acquisitions must contend with the increased venous signal intensity as the contrast agent continues to distribute in the system. In addition, the expected use of intravascular contrast agents with much longer persistence will require more novel techniques for arterial-venous discrimination.
Phase contrast magnetic resonance angiography (MRA) is a practical and clinically applicable technique for imaging blood flow. MRA makes use of flow encoding gradient pulses which impart a velocity-dependent phase shift to the transverse magnetization of moving spins while leaving stationary spins unaffected. Each phase contrast acquisition generates two images: a magnitude image that is proportional to the proton density of the object and may also be T.sub.1 -weighted, and an image representing the phase of the object. The phase image produced has information only from the moving spins and the surrounding stationary tissue is suppressed. Images representing both the average flow over the entire cardiac cycle and at a series of individual points in the cycle have been generated using this technique. The phase contrast MR method produces phase images with intensities that represent the magnitude of the flow velocity and also the direction of flow. Therefore, such images may be used for both qualitative observation of blood flow and quantitative measurement. The practical application of phase contrast MR angiography and venography to the quantitative determination of flow velocity is therefore evident.
In a phase contrast image, the phase shifts generated in a pixel by moving spins is directly proportional to the motion of the ensemble of spins. If the velocity is linear, then the phase shift is directly proportional to the velocity and the sign of the phase shift is indicative of the direction of flow. As phase representation are in terms of an angle, unique values of flow velocity and direction of motion can be obtained if the phase shifts are constrained between .+-..pi.. That is, the velocity encoding value or VENC, is given by: ##EQU1##
where .gamma. is the gyromagnetic ratio, and .DELTA.M.sub.1 is the gradient moment and is proportional to the area of the flow encoding gradient waveform. This value of VENC is such that all flow velocities will have values constrained between .+-..pi.. The noise level in the phase image is also proportional to the velocity encoding value. It can be shown that the noise level in the phase image, .sigma..sub.v, is related to the VENC value by: ##EQU2##
where M is the magnitude of the spins in a voxel, and .sigma. is the noise variance of the acquisition. Thus, by raising the VENC value, the noise level in the phase image increases correspondingly.
In one prior art time-of-flight MRA technique that gates the acquisition to systole and diastole to generate an image with enhanced arterial flow in systole, a second image is provided with less arterial flow enhancement in diastole. The venous flow in both images would be approximately the same, however. See Pulmonary Vasculature: Single Breath-Hold MR Imaging With Phased Array Coils. Radiology 1992; 183: 473-477, Foo T K F, MacFall J R, Hayes C E, Sostman H D, and Slayman B E. By subtracting the two images, the common venous flow mode can be eliminated, together with the stationary background noise. However, such a technique is not practical and patients with peripheral artery disease where the difference in flow between the systole and diastole is not significant. Other techniques which attempted to distinguish between arterial and venous flow require imaging during a first pass of the contrast agent, and then subtracting the images from the initial arterial phase from a latter venous or equilibrium pass. Such techniques have been found to be too dependent upon the unpredictable flow of the contrast agent in the patient.
It would therefore be desirable to have a method and apparatus for MRA that efficiently discriminates between arterial and venous signals to create a venous only image or an arterial only image using magnetic resonance imaging.